Louisiana specialist says: Minimize stresses on soybean crop

Mid-January, the commodity market is demanding soybeans. As they solidify planting intentions, U.S. farmers are surely paying attention.

However, after a poor late summer in 2007, there is a serious dearth of quality soybean seed. That development has seed companies scrambling to cover orders.

Meanwhile, winter meetings in the Mid-South have given voice to those nervous about producing the needed acreage and warnings about making the first stand count. A lack of seed means replanting must be kept to a minimum.

In hopes of that, David Lanclos said efforts to ward off crop stresses should be vigorous. Heat stress late last summer, the LSU AgCenter soybean specialist said, is largely responsible for poor seed and the spate of “green beans” that refused to dry down when mature.

“Plants that are stressed will often perform a last-ditch effort to reproduce, including rapid and profuse blooming followed by setting seed,” Lanclos told those at the Louisiana Soybean Association annual meeting in Alexandria, La.

His definition of stress included: “When a plant or crop is adversely affected due to any single and/or combination of environmental conditions causing a slowdown or reversal in normal photosynthetic processes which then causes the plant or crop to reboot in an attempt to make seed at all costs.”

How serious a problem are stresses on a soybean crop?

“Because of stresses — among them temperature and heat, drought and water logging, mineral deficiency and mineral toxicity, salinity, weeds, insects and diseases — the yield of field-grown crops in the United States is only 22 percent of genetic potential.”

Lanclos pointed to several stresses as especially problematic in recent growing seasons. High temperatures led the list.

“Optimum crop growth rate is achieved once temps reach about 86 degrees. Temperatures can rise into the high 90s without adversely affecting the optimal growth rate.”

If temperatures rise higher, however, “stress may set in, depending on how hot it gets and for how long. Just 15 to 30 minutes of 110 to 112 degrees can cause enzyme denaturation and/or injury to photosynthetic membranes.”

Last August, temperatures were hellish just as crops around central Louisiana reached R-6. During the third week of the month, temperatures remained well above 100.

Whether too much or too little, water can be a great stress on soybeans. If in deficit, water demand “is set by the plant transpiration rate or crop evapotranspiration, which includes both plant transpiration and soil evaporation.”

Meanwhile, in waterlogged soils, “diffusion of gases through soil pores is so strongly inhibited by their water content that it fails to match the needs of growing roots. A lowing of oxygen influx is the principal cause of injury to roots and the shoots and above-ground materials they support.”

Some 65 percent of water use occurs between R-1 and R-6.

Lanclos said an Iowa State University study on “four days of visual moisture stress on a soybean crop” displays how critical it is to keep soybeans from water stress. Stress the first week of flowering causes an 8 percent yield decrease. Stress during the first week of pod development drops yields 19 percent. Stress during the first week of seed fill or the third week of pod development can drop yields 36 percent. Stress during the second to fourth week of seed fill can lower yield 39 to 45 percent.

Managing weeds, insects and diseases are also a key to growing good soybeans. “It's been documented extensively that if weeds are controlled in the first 28 days of growth, yield is enhanced.”

To illustrate the need for early control, Lanclos pointed to a three-year test around Urbana, Ill. The study looked at “seven common cocklebur accessions from different regions of the United States. Plots were established with each common cocklebur plants between soybean rows at a density of one plant per meter of row. Averaged across all locations, soybean yield reductions were 48 percent and 20 percent in 1999 and 2000, respectively.”

A 2005 stink bug study — looking at infestations on a Group 5 variety for two weeks at various growth stages — is also eye-opening. Two weeks of light-to-heavy infestations at R-3/R-4 led to yield losses between 3 percent and 42 percent. Two weeks at R-5 led to losses between 45 percent and 51 percent. Two weeks at R-6 led to losses between 18 percent and 26 percent.

There are things producers can do to decrease the chances of stress on soybeans. Among them:

Start with a proven variety.

Make sure soils have proper fertility.

Use correct plant population and seeding depth.

Use correct row spacing (under 30 inches, or twin).

Employ a seed treatment.

Plant in the correct window to maximize yield and avoid late-season stresses. “This one can be tricky.”

Be aggressive with weeds early by using a pre followed by early shots of glyphosate and a tank-mix partner, if necessary.

Be aggressive with early scouting for insects.

Be aggressive in scouting for diseases. Spray the right fungicide at the correct growth stage (R-3/R-4) to get the most bang for the money.

Make sure fields drain properly.

Be prepared to apply Gramoxone Inteon at R-7.

The bottom line, said Lanclos is “there's rarely a single contributing factor that's caused the crop to suffer. Rather, it's more common for irreversible stresses to have been caused by a number of factors that have actually occurred simultaneously.”